In the discussion of the background that follows, reference is made to certain structures and/or methods. However, the following references should not be construed as an admission that these structures and/or methods constitute prior art. Applicant expressly reserves the right to demonstrate that such structures and/or methods do not qualify as prior art.
Large trenching machines utilize large plates linked around a boom to form a cutting chain. Blocks to hold the tool picks are welded at the rear of these plates for optimal cutting. FIGS. 1 and 2 show an example of such a plate 10 with a block 12 fixed to a surface 14 with a weld 16. Located in the block 12 is a tool pick 18. Conventional tool picks utilize rear retainers 20 where a portion of the shank of the tool pick 18 extends past the rearward surface 22 of the block 12.
During operation as the chain of long plates travel around the boom to excavate material, the plates 10 pivot. FIG. 3 illustrates a portion of the boom 30 indicating travel direction D of the plates 10. During an excavating portion 32 of the travel cycle, the linked plates 10 are spaced apart due to friction with the material being excavated; during a return portion of the travel cycle, the linked plates 10 are slack and in some locations the plates 10 bend towards one another, reducing the clearance therebetween. In such a circumstance, the edge of the exposed end 24 of the shank of the tool pick 18 can be jammed against the following plate 10, applying a force to the end 24 that can be high enough to deform the exposed shank and/or the rear retainer 20 of the tool pick 18. Also, the contact point 26 on the plate 10 can itself be deformed (see FIG. 2). The deformation can cause the edge of the exposed end 24 to misshapen, for example to spread out into a shape similar to a mushroom, or other portions of the tool pick 18 to misshapen. In such cases, removal of the tool pick 18 through the bore of the block 12 can be complicated and/or prevented. Where used, removal of a rear-retainer or employment of special tools for retainer removal or tool pick removal are likewise complicated and/or prevented by portions of the tool pick being misshapen.
Wear is detrimental to the lifetime performance of assemblies. For example, to encourage pick rotation, there is typically clearance between the pick and its sleeve. However, this clearance allows space for dust and fines to collect between the tool pick and the bore of the block. As rotatable elements of the assembly rotate, this material grinds the opposing elements, thereby enlarging the bore and allowing more fines to enter, accelerating the wear and reducing the life of the block. Similar wear problems can occur between the sleeve and one or more of the bore of the block or the shank of the tool pick if the clearance is too large and dust and fines collect therebetween. With regard to the use of an external retainer, a certain amount of clearance is required between the rear of the block and the groove in the pick shank to assemble the retainer. If too large, this clearance allows unnecessary freedom of movement between the pick and block, causing an unwarranted amount of slapping between the pick shoulder and face of the block. This slapping causes excessive wear in the bore and on the face of the block, reducing its life.